Hard disk drives have been in use for nearly twenty years and are found in most computers, as well as other types of electronic systems where non-volatile recording of large quantities of digital data is required. Many techniques have been developed over the years to improve the performance of hard disk drives, including magneto-resistive read/write heads, various RLL encoding schemes and the now well-known if not ubiquitous PRML (partial response maximum likelihood) read channel. One of the important improvements adopted early on in disk drive evolution was the "servo wedge"--areas on the magnetic recording surface where timing and control information are stored, generally interspersed between encoded user data storage regions. For example, a typical data track on a hard disk includes a series of spaced-apart data regions, alternating with embedded servo regions. The data regions are typically encoded according to a PRML code specification. There may be on the order of 100 or more servo regions embedded within each concentric data track, as further explained later with reference to the drawings. Each servo region includes information used by the head position servo for precisely positioning and tracking the head over the particular track. Each servo region also includes a "preamble pattern," typically a repeating pattern of plus and minus magnetic flux transitions that will reproduce as a sine wave analog signal in the read channel for the purpose of synchronizing the servo demodulator phase locked loop (PLL) to the servo position data stored on the recording surface. Interspaced with the servo wedges are data sectors where the digital data is stored on the magnetic media. Each data sector likewise includes a preamble sine wave pattern that is used for synchronizing the read channel PLL to data being read off the magnetic disk. A representative disk drive that employs the technologies summarized above is described in detail in commonly-assigned U.S. Pat. No. 5,345,342 entitled "DISK DRIVE USING PRML SYNCHRONOUS SAMPLING DATA DETECTION AND ASYNCHRONOUS DETECTION OF EMBEDDED SECTOR SERVO."
At the beginning of a disk read operation or, more precisely, in preparation for each data read operation or servo operation, the read channel timing loop circuitry is re-synchronized to the current preamble pattern. In prior art, this timing acquisition is aided by a combination of either (1) adaptive or two-stage phase-locked loop (PLL) filters; and/or (2) analog Zero-Phase Restart (ZPR) techniques, according to which the voltage-controlled oscillator (VCO) is held for a short time, and then released so as to be aligned with the incoming analog read signals.
Adaptive PLL loop filters improve acquisition performance by increasing the loop bandwidth for a short duration to achieve phase lock; at the cost of increased phase jitter. Then the loop bandwidth is reduced to the steady state value required to meet allowed jitter tolerance. The acquisition improvement is a function of the size of the increase in the loop bandwidth, which in turn is limited by the size of disturbance that the PLL can tolerate without completely losing synchronization.
Analog ZPR circuits function by holding the analog VCO for a short time, and then releasing the oscillator so as to align the VCO phase with the input signal phase. Analog ZPR accuracy is limited by the tolerances of the analog circuitry. The critical tolerances that affect accuracy include that of the sensor that detects the input signal phase; the hold-off circuit that stops the VCO; and the reaction delay time of the controlling circuitry. All of these tolerances conspire to limit the accuracy achievable using the analog hold technique. In view of the foregoing background, the need remains for improvements in rapidly acquiring synchronization to the preamble pattern in a disk drive read channel. Acquisition performance is critically important in a magnetic read channel because the time spent in acquiring or synchronizing to the input signal reduces the time that could otherwise be spent reading user data off the disk. And similarly reducing the size of the preamble pattern translates to better utilization of the recording surface area for user data.